Structure of DNA

Topics covered:

  • nucleotide structure
  • DNA structure
  • pairing of bases
  • the double helix
  • function of DNA
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  • Created by: Charlotte
  • Created on: 14-03-12 22:52

Nucleotide Structure

  • individual nucleotides of DNA are made up of three compounds:
    • deoxyribose (a sugar)
    • phosphate group
    • an organic base belongin to one of two different groups:
      • single-ring bases - cytosine (C) and thymine (T)
      • double-ring bases - adenine (A) and guanine (G) (http://t0.gstatic.com/images?q=tbn:ANd9GcQwRxQkCpaedPuG9MJ9wsJCvMMCLW7AKIYl2Q3gWQfg7opOO4tI)
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DNA Structure

  • made up of two strands of nucleotides (polynucleotides)
  • each strand is joined together by hydrogen bonds formed between bases.
  • 1953 the structure was worked out by James Watson and Frances Crick  (http://t0.gstatic.com/images?q=tbn:ANd9GcSWMvCokqmGH3YPZmzjnNkeQ6TxHAIKuO37aTr_nFys84DxT-gJ)
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Pairing of the bases.

  • Organic bases contain nitrogen and are one of two types:
    • Double-ring structure (adenine and guanine)
    • Single-ring structure (cytosine and thymine)
  • Those with a double-ring structure have longer  molecules than those with a single-ring structure 
  • the base pairs must always be make up of one of each type, more specifically;
    • Adenine always pairs with thymine by means of 2 hydrogen bonds
    • Guanine always pairs with cytosine by means of 3 hydrogen bonds
  • This means A is complementary to T and G is complementary to C.
  • The ratio of A and T to G and C varies from species to species. 
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The double helix.

  • The uprights of phosphate and deoxyribose wind around one another to form a double helix.
  • For each complete turn of the helix there are ten base pairs(http://t3.gstatic.com/images?q=tbn:ANd9GcT4o1gcNEUJm__1J0K5IiZp4EP_ajYgW85ocZUYVnSc32QQntGApg)
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Function of DNA.

  • DNA: the hereditary material responsible for passing genetic information                from cell to cell, generation to generation.
  • There are around 3.2 billion base pairs in a typical mammalian cell.
  • This means there is almost infinite variety of sequences of bases along the length of a DNA molecule.
  • This provides the immense genetic diversity within living organisms.

The DNA molecule is adapted to carry out its functions in a number of ways:

  • It is very stable and can pass from generation to generation without change
  • Its two strands are joined only by hydrogen bonds, which allow them to separate during DNA replication and protein synthesis.
  • It is a large molecule so carries a lot of genetic information.
  • The base pairs are within the helical cylinder of the deoxyribose-phosphate "backbone" and so the genetic information, to some extent, is protected from corruption by outside chemical and physical forces.
  • DNA depends on the sequence of base pairs that it processes.
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